The present invention relates to a secondary-ion mass spectrometry (SIMS) apparatus, and more particularly to a secondary-ion mass spectrometry apparatus using a field limiting method in which only ions generated from a sample portion to be analyzed are introduced into a mass spectrometer portion of the apparatus, thereby making it possible to acquire analysis data having high reliability.
A secondary-ion mass spectrometry apparatus is one in which secondary ions emitted from a sample surface by irradiation thereof with a primary ion beam are introduced into a mass spectrometer portion of the apparatus to make mass-spectrometric analysis of the secondary ions. When the sample surface is scanned with the primary ion beam, constituent elements of the sample surface are etched with the lapse of time, thereby allowing an elemental or component analysis of the sample in a depth direction thereof. This is disclosed by U.S. Pat. No. 3,894,233.
In the case where a sample is a conductor, charges produced on a surface of the sample are dissipated through the conductor sample even if the reception and release of charges including the irradiation of the sample surface with a primary ion beam and the emission of secondary ions and secondary electrons from the sample surface are made at the sample surface. However, in the case where the sample is an insulator, charges produced on the sample surface residue (or are accumulated) thereat and hence the sample surface is charged. Therefore, the energy of secondary ions emitted from the sample surface changes depending upon the charges accumulated on the sample surface. When such secondary ions are introduced into a mass spectrometer portion, the accuracy of mass-spectrometric analysis of secondary ions is affected.
A method of avoiding the charging phenomenon at the sample surface affecting the mass-spectrometric analysis of secondary ions including an electron spray method disclosed by JP-B2-54-6912. When a sample surface is irradiated with a primary ion beam so that secondary ions and secondary electrons are emitted from the sample surface, the sample surface is generally charged with positive charges. According to the electron spray method, the sample surface is irradiated with electrons in order to neutralize the positive charges with which the samples surface will otherwise be charged.
Another method of avoiding the charging phenomenon at the surface of an insulator sample is an electron bombardment induced conductivity (EBIC) method disclosed by Y. Ikebe et al. Seventh Int. Conference on SIMS, (1989) pp. 891-894. In the EBIC method, the sample surface is irradiated with electrons on the basis of the same principle as the electron spray method to form an electrically conductive layer on the sample surface in order that charges accumulated on the sample surface are dissipated through the conductive layer.
The two methods mentioned above are effective to the secondary-ion mass spectrometry apparatus but involve the following problems.
In the electron spray method, though it is intended to avoid the charging of the surface of an insulator sample by irradiating the sample surface with electrons, it is not possible to sufficiently neutralize charges accumulated on the sample surface. Further, no means for confirming the degree of neutralization is provided.
In the EBIC method, the conductive layer formed by the EBIC method may be extinguished as the analysis of the sample surface for layers to be analyzed along a depth direction progresses with the lapse of time. As a result, a charging phenomenon may be generated again at the sample surface or the charging state of the sample surface may change. No means for quantitatively discriminating such a change of the charging state is provided.
Namely, in the above-mentioned insulator analysis methods used in the conventional SIMS apparatus, no consideration is taken for (1) the provision for confirming the avoidance of charging and (2) the provision for quantitatively discriminating the change in charging state. Accordingly, in the analysis of insulator by the prior art, it is not possible to quantitatively grasp the charging state and hence the reliability of data acquired is greatly questionable.
As a further prior art is known a secondary-ion mass spectrometry apparatus using a field limiting method disclosed by H. Tamura et al. "DEVELOPMENT OF A NEW FIELD LIMITING METHOD FOR SIMS", SIMS VII, (1989) pp. 903-906. In the case where the mass-spectrometric analysis of constituent elements of a sample surface is conducted by scanning the sample surface with a primary ion beam while deflecting the beam, the analysis progresses to the constituent elements of a more deep layer of the sample gradually with the lapse of time. Generally, in that case, secondary ions emitted from the vicinity of the edge portion of a sample surface scanning region of the primary ion beam originate from a layer shallower than a layer from which secondary ions are emitted from the vicinity of a central portion of the scanning region. Therefore, the accuracy of component analysis of each layer for the depth direction of the sample becomes poor. According to the field limiting method, secondary ions emitted at the edge portion of the sample surface scanning region of a primary ion beam are intercepted and only secondary ions emitted at the central portion of the scanning region are introduced into a mass spectrometer portion of the apparatus.